After three decades of study, Gamma-ray Burstsstill mystify

After three decades of study,
gamma-ray bursts still mystify

Interview with NASA/Marshall's Jerry
Fishman

Oct.
15, 1999: For more than three decades, gamma-ray
bursts have been one of the most mysterious phenomena in astrophysics.
While not as popular as black holes, they have nonetheless stirred
public interest. Next week, more than 200 of the world's astrophysicists
will gather in Huntsville for the 5th biennial Huntsville Gamma
Ray Burst Symposium. Science@NASA caught up with Dr. Gerald Fishman
(right), the principal investigator for the Burst and Transient
Source Experiment, for an interview about bursts and the symposium.

Science@NASA: Tell us about the 5th Huntsville Gamma
Ray Burst Symposium. Who will be attending and what they expect
to get out of the symposium?

Fishman: This is the 5th in a series of gamma-ray bursts
symposia that we have here in Huntsville. Actually, researchers
from all over the world descend upon Huntsville because Huntsville
is the source of the primary data base of gamma-ray bursts and
has been since the launch of the Compton Gamma Ray Observatory.
So it's an appropriate place to hold such a conference.

Science@NASA: What are gamma-ray bursts, how were they
discovered and how are we studying them?

Fishman: Gamma rays are the highest energy form of
radiation. They are higher energy than X-rays; they are very
penetrating. They'll go through several inches of steel, for
example. So you need very large, massive detectors or instruments
in order to stop the gamma rays and to detect them. The Gamma
Ray Observatory was launched in 1991 by the Space Shuttle to
detect various gamma rays coming from different objects in the
universe. These are the highest energy objects that we know of
in the universe -- things like black holes, neutron stars, and
gamma-ray bursts.

The gamma-ray bursts were discovered by accident
over 30 years ago. No one ever suspected these immense, long
flashes of gamma rays coming from different parts of the sky.
The United States orbited some nuclear bomb detection satellites
to look for clandestine nuclear explosions that might occur in
deep space. Of course they didn't find any, but they started
seeing little sporadic flashes or bursts of gamma rays at different
times. When they started to correlate these among satellites,
they saw they were coming in coincidence with each other and
that they were coming from different directions in space. They
weren't coming from the Earth, and they weren't coming from the
sun or any other known objects in the solar system. But they
were so unsure of themselves, it was such an unexpected discovery,
that they waited four or five years until they were really sure
they had something new before they announced the discovery in
1973.

Science@NASA:
What are the primary instruments that we have for studying bursts
now?

Left: An artist's concept of a Vela satellite in orbit.
Credit: TRW.

Fishman: Since the time after the Vela satellites first
discovered gamma-ray bursts, there have been different instruments
in the 1970s and '80s that detected gamma-ray bursts. However
it hasn't been until the 1990s that very large dedicated experiments
to specifically study the gamma-ray bursts have been launched.
The foremost among these is the BATSE experiment, the Burst and
Transient Source Experiment on the Compton Gamma Ray Observatory
that was launched in 1991. Because it has such large detectors
and was launched aboard the largest scientific spacecraft ever
put into orbit by the U.S., it has a very large sensitivity,
high sensitivity to gamma rays. It can detect very faint and
therefore very distant gamma-ray bursts.

Science@NASA: You mentioned that gamma-ray bursts
have uncommonly high energies that can penetrate several inches
of steel. Obviously conventional telescopes will not work with
these. How do we detect them? What is the mechanism?

Fishman: In order to detect gamma rays you need a very
special type of telescope. They need to be heavy and thick and
they need to somehow convert the gamma rays into another form
of radiation that can be more easily detected. For this we use
scintillation crystal detectors. A scintillation crystal has
a property that when radiation hits it, it gives off a little
flash of light or what we call scintillation. These faint flashes
of light are then detected by other optical detectors. Then the
pulses are transmitted to the ground.

Science@NASA:
How many bursts have we observed with BATSE and what have we
learned from the eight-plus years of observation with it?

Fishman: Initially gamma-ray bursts, because they were
studied with only small detectors, were only seen about 10 to
20 times per year. Since the launch of the BATSE experiment we've
been seeing about one a day or 300 per year. We now have a catalog
of over 2,500 gamma-ray bursts which is over half of the gamma-ray
bursts ever seen since their discovery.

Left: Artwork for 5th Huntsville Gamma Ray Burst Symposium
symbolizes the brightness profiles of bursts and their distribution
across the sky. Credit: NASA/Marshall.

We've been able to characterize the various properties of
these gamma-ray bursts -- that is, their distribution over the
sky, their intensity distribution, how the different energies
of gamma rays are spewed forth.

From these observations come many theories --Â in fact,
there are over 150 theories -- of what causes a gamma-ray burst.
People are starting to settle in on a couple of specific ideas.
It wasn't until after the BATSE experiment was launched that
we realized that gamma-ray bursts were at the distant edges of
the universe. Before that most people thought that they were
relatively nearby in our own galaxy. Because they are so distant,
they have a tremendous of energy. In fact, they are the largest
known explosions in the universe, thousands of times brighter
than supernovae. But in recent years, it has occurred to several
people that supernovae and gamma-ray bursts may be somehow related.
Because we know that both objects come from a compact small region
and they somehow involve explosions.

Many people think that this is due to the collapse of a supermassive
star, or perhaps the combination or merger of two very dense
compact objects such as neutron stars or a neutron star and a
black hole. This would lead to a tremendous outpouring of energy.
For example, if you had a marshmallow fall into a neutron star,
it would produce as much energy as a thousand hydrogen bombs.
What we are talking about here a significant part of a star falling
onto a neutron star. This would cause an enormous explosion.
In fact, the amount of energy that they put out is more energy
than the sun puts out in its entire 10 billion-year lifetime.

Science@NASA:
From what we know, most of these are very distant in the universe
and therefore very old in time. Is this a phenomenon that we
are likely to see happen in our galaxy or has already happened
in the distant past?

Fishman: The best estimate of how often gamma-ray bursts
occur is about once every hundred million years. Since our own
galaxy is probably 10 or 15 billion years old, yes, many of them
have happened in our own galaxy. In fact, some people think that
perhaps that was the reason the dinosaurs became extinct maybe
65 million years ago, because a nearby gamma-ray burst disturbed
the Earth's atmosphere and caused the dinosaurs to die.

Science@NASA: But there has also been some thinking
that perhaps the shock waves from gamma-ray bursts could even
play a role in planetary formation?

Fishman: A recent paper has speculated that perhaps
a very large explosion is what compresses the interstellar medium
to actually form planets. It is a very speculative thought but
an interesting one. Because others that have tried to look into
planetary formation have had trouble condensing the very diffuse
interstellar matter, it could it be that just such an explosion
might help condense interstellar material and form new planets.

Fishman: Gamma rays are given off by the most exotic
and energetic objects in the universe. BATSE can study these
as well as gamma-ray bursts. Because we have eight BATSE detectors
and because we look at the entire sky, we're sensitive to any
transient or explosive object or a large flare or flickering
from known objects. Examples of things that we've seen are black
holes within our own galaxy, neutron stars, binary neutron star
systems, pulsars, solar flares and very interesting objects known
as microquasars, which are believed to be black holes within
our galaxy that spew out very narrow, collimated jets, very fine
jets of material close to the speed of light. These produce gamma
rays as well.

Science@NASA: But among these are the most intriguing
- microquasars sound neat - but the most intriguing, and certainly
the most newsworthy over the last year and half are the magnetars.

Fishman: It's been known for some time that neutron
stars have very strong magnetic fields. But it is only recently,
the past year and a half, that it has become recognized that
there is a certain class of neutron stars we call magnetars that
have an extraordinary strong magnetic field. This magnetic field
is so strong that when a perturbation - a starquake - in a neutron
star occurs, this intense magnetic field is shaken and it accelerates
particles to very high energies and we see this as an enormous
blast of x-rays and gamma rays.

But there are only four magnetars that are known now. Three
of them are in our own galaxy and one of them is in a neighboring
galaxy, the Large Magellanic Cloud. The magnetar theory is very
interesting because it predicts types of radiation and phenomena
that we can't possibly study here on Earth because of the magnetic
fields of a magnetar are billions of times stronger than anything
we can create in a laboratory.

Making a neutron star - and a magnetar
- starts (1) with a massive star that has burned up all of its
fuel, then (2) collapses and causes a massive explosion, the
supernova that blows off the outer layers and (3) compresses
the core. Soon, all that is left is a shell of expanding gas
(not always this pretty or symmetrical) and a rapidly spinning
neutron star at "ground zero." If the original star
was spinning fast enough and had a strong enough magnetic field,
the neutron star is a magnetar.

Science@NASA: And one last surprise that BATSE gave
us apparently comes from thunderstorms?

Fishman: Yes, this was a complete surprise. Probably
the last place we ever expected to see gamma rays coming from
is the Earth itself. But soon after launch, about once a month,
we would see a very brief flash, an intense flash of gamma rays,
lasting only a few thousandths of a second. When we analyzed
the data we found they were coming from the Earth near large
thunderstorm systems on the Earth. This was completely unexpected.

At the same time other people have been studying what are
called sprites. This is some sort of electrical discharge phenomena
that occurs between the tops of thunderstorms and go upward all
the way through the stratosphere to the ionosphere. It is thought
that the same conditions that are capable of producing these
beautiful red optical sprites are also capable of producing gamma
rays. However it is still speculative.

Science@NASA: And finally what is the next step beyond
Compton and beyond BATSE?

Fishman: Well the real key to making progress in gamma-ray
bursts is to identify where the bursts occur very precisely so
you can look at the host galaxy. That is what kind of a galaxy
it was formed in, and what the environment of the gamma-ray burst
source is like. A breakthrough came about three years ago from
the Italian-Dutch satellite Beppo SAX. For the first time, they
were able to precisely and quickly get the direction to a gamma-ray
burst so that x-ray and optical follow-up observations could
be made. Ever since gamma-ray bursts were discovered, people
thought what we really needed was to find the counterpart, that
is to study a gamma-ray burst in some other part of the electromagnetic
spectrum.

BATSE
can't locate the gamma-ray bursts very precisely so it was up
to the Italian-Dutch satellite, Beppo SAX, to do that job. They
discovered that, sure enough, there was x-ray and optical radiation
that lasted for several days or even weeks after the initial
explosion. The Hubble Space Telescope has found very faint but
positive afterglow emission coming from some of these gamma-ray
bursts locations as much as six months after the initial explosion.

Left: Computer animation depicts a wave of radiation
spreading across the universe, and a small portion being detected
by BATSE. Credit: NASA/Marshall.

This has led to a revolution in the development of models
as to what's causing the gamma-ray bursts and how the radiation
comes out as what is called a fireball, a relativistic blast
wave of radiation that slams into the nearby interstellar medium.
Because gamma-ray bursts are so far away we can actually use
gamma-ray bursts to study the most distant and therefore the
earliest regions of the universe. We expect they are going to
become as useful for these early universe studies in cosmologies
as supernova have become. So the push in the future is to get
more satellites up there that can precisely locate gamma-ray
bursts and provide this information to astronomers that use ground-based
and space-based telescopes.

The next such mission will be launched in January. It is called
the HETE II mission, led by a group of scientists at MIT. Following
that we expect to hear very shortly that a new mission has been
approved called SWIFT. SWIFT has a variety of telescopes on board
as well as gamma- ray detectors that will be able to quickly
respond to gamma-ray bursts. And then perhaps ten years from
now we expect a major facility called the Next Generation Gamma
Ray Burst Observatory to provide us with perhaps thousands of
very precise gamma-ray burst observations with good locations.